The utilization of a helicopter frequently involves the capturing of a trim position to lock in on aircraft position, speed, altitude, attitude, etcetera. Capturing these trim values is performed by autopilot and automatic flight control systems which include synchronizers to store (i.e., capture) the desired trim. As an example, if a pilot desires to maintain a certain velocity/aircraft attitude, he would capture the velocity/attitude trim by pushing a button on the collective stick, which triggers a synchronizer to store the present value it is receiving, which is indicative of the desired trim position.
Once the trim position has been stored, the output of the synchronizer instantaneously goes to zero, which acts to normalize set points in the autopilot and flight control system with respect to stored trim position. The synchronizer then provides a signal indicative of the difference between the stored trim position, and the current value of the input signal.
The problem with this synchronizer is that its output goes to zero instantaneously when commanded to store the trim value. This instantaneous change causes a transient aircraft response which gives the pilot the perception the aircraft is responding in the wrong direction. Providing such an erroneous perception to the pilot is clearly unacceptable.